1. Field of the Invention
The present invention relates to a driving circuit for a vacuum fluorescent display.
2. Description of the Related Art
A vacuum fluorescent display (hereinafter, referred to as “VFD”) is a display device of a self-illuminating type for displaying a desired pattern by causing a direct-heating type cathode called a filament to emit thermoelectrons by causing it to generate heat by applying a voltage thereto in a vacuum chamber and by causing the thermoelectrons to collide against fluorescent material on an anode (segment) electrode and causing them to illuminate, by accelerating the thermoelectrons using a grid electrode. VFDs have excellent features in terms of visibility, multi-coloring, low operating voltage, reliability (environmental resistance) etc. and are used in various applications and fields such as cars, home appliances and consumer products.
Here, for a VFD, in the case where a short circuit or wire-breaking has occurred to the filament or its wiring, where a short circuit has occurred between the wiring of the filament and the wiring of another electrode(such as the anode electrode or the grid electrode) or where an element for driving the filament has had a failure, there arises risk of causing damage to the filament or ignition of the filament when the abnormal state of the filament is left as it is. Therefore, a mechanism for detecting immediately such an abnormal state of the filament is sought for the VFD.
FIG. 9 illustrates a conventional mechanism for detecting an abnormal state of a filament voltage applied to a filament 11 as one of the mechanisms as described above. The figure shows an example in which, as a scheme for applying a voltage to the filament 11, “pulse-driving scheme” in which a pulse voltage (hereinafter, referred to as “filament pulse voltages”) produced by chopping a considerably higher DC voltage compared to the ordinary nominal voltage of the filament is applied is employed. That is, in the pulse-driving scheme, the progress of the damage to or the ignition of the filament 11 is faster compared to that in other schemes (such as DC-driving scheme and AC-driving scheme) in case an abnormal state such as that the filament pulse voltage is fixed to the higher potential side has occurred. Therefore, it is important to detect immediately the abnormal state of the filament pulse voltage.
In FIG. 9, an external controller 40 such as a microcomputer outputs a pulse-driving signal with a duty ratio set at a desired ratio to a filament driving circuit 110. Then, the filament driving circuit 110 generates a filament pulse voltage from a power source for the filament 11 by a switching operation based on the pulse-driving signal received from the external controller 40, and applies the filament pulse voltage to the filament 11.
Here, the external controller 40 comprises a detecting unit for detecting, for example, the pulse width or the voltage level of the filament pulse voltage for the filament pulse voltage applied to the filament 11.
The external controller 40 executes feed back control in which the settings of the duty ratio of the pulse-driving signal that the external controller 40 outputs to the filament driving circuit 110 are adjusted in response to the pulse width or the voltage level of the filament pulse voltage detected by the detecting unit.
The mechanism described above is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 2002-108263.
In the conventional mechanism for detecting an abnormal state of the filament pulse voltage, the external controller 40 detects the pulse width or the voltage level of the filament pulse voltage and desired feedback control is executed to the filament pulse voltage in response to the detected values. However, this is also a factor for increasing the load of processing on the external controller 40. Furthermore, the external controller 40 has a problem that it needs considerable time from the moment at which it detects an abnormal state of the filament pulse voltage to the moment at which it executes a predetermined response (for example, turning off the power of the filament driving circuit 110), due to the increase of the load of processing in itself and, therefore, leads to the damage or the ignition of the filament 11.